A liquid crystal display (LCD) module typically includes a backlight module and an LCD panel. The backlight module includes light sources, such as light emitting diodes (LEDs) that emit red light, green light, and blue light. The LCD panel displays an image using the three color light sources. The LCD panel also includes a color filter, an active device array substrate, and a liquid crystal layer disposed between the color filter and the active device array substrate.
Light from a light source first passes through the active device array substrate, which has an array of pixel electrodes and associated active devices such as thin film transistor (TFTs). The active device array substrate receives pixel signals for controlling voltages of the pixel electrodes to adjust orientations of liquid crystal molecules in the liquid crystal layer for controlling the intensity of light passing through the liquid crystal layer. The light eventually produces a corresponding color on the LCD panel due to the filtering function of the color filter if the light passing through a particular pixel electrode can smoothly penetrate the liquid crystal layer under the control of the pixel electrode voltage.
FIG. 1 is a graph illustrating the emission spectra of corresponding LEDs and the transmission spectra of corresponding color filters of a typical LCD module. In FIG. 1 “LCFR” represents the transmission spectrum curve of a low purity red filtering layer in a conventional color filter, “LCFG” represents the transmission spectrum curve of a low purity green filtering layer, and “LCFB” represents the transmission spectrum curve of a low purity blue filtering layer. Each transmission spectrum curve represents a respective range of wavelengths of light that is allowed to pass through the corresponding filtering layer. “LEDR” represents the emission spectrum curve of a red LED, “LEDG” represents the emission spectrum curve of a green LED, and “LEDB” represents the emission spectrum curve of a blue LED. The transmission spectrum of a light source such as an LED represents a range of wavelengths of light emitted by the light source.
Generally, the intensity or penetrability of light passing through the color filter is high if a low purity color filter is used. However, as shown in FIG. 1, besides filtering the light emitted by the corresponding color LED, the transmission spectrum of a particular filtering layer may overlap with the emission spectrum of another color LED, so that it is possible that the light emitted by the other color LED may also radiate through this particular filtering layer. For example, the transmission spectrum curve LCFG of the low purity green filtering layer has a wavelength width that partially overlaps with the emission spectrum curves of LEDB and LEDR (spectrum curves for the blue and red LEDs). This overlap may adversely affect the light filtering effect in the color filter.
In FIG. 1, “HCFR” represents the transmission spectrum curve of a high purity red filtering layer in another conventional color filter, “HCFG” represents a transmission spectrum curve of a high purity green filtering layer thereof, and “HCFB” represents the transmission spectrum curve of a high purity blue filtering layer thereof. To improve the poor light filtering effect described above when low purity filtering layers are used, the purities of the three color filtering layers of the conventional color filter can be increased so that the width of the high purity color filter transmission spectrum and the width of the LED emission spectrum do not partially overlap as is the case with low purity color filtering layers. As a result, when using high purity color filter layers, the light filtering effect of the color filter is improved.
FIG. 2 is a CIE chromaticity diagram of a conventional LCD color filter. It can be seen from FIG. 2 that by increasing the purities (in FIG. 2, purity “HCF” is greater than purity “LCF”) of the three color filtering layers of the color filter, the displayed NTSC (National Television System Committee) ratio of the LCD module is increased. The NTSC ratio refers to the ratio of a color area of a color filter to the color area of NTSC.
Although high purity color filtering layers can be used to improve the filtering effect of a color filter, using high purity filtering layers may lead to higher manufacturing costs and decreased light penetrability of the color filter (which in turn reduces the display brightness of the LCD). Thus, even though the current design trend toward higher purity color filtering layers may somewhat increase display quality, manufacturing cost is increased greatly, so that the relative benefit of using higher purity color filtering layers is rather limited.